During the peak of the Kaikoura Orogeny, rain, rivers,
wind, and waves were joined in their attack upon the rising
land by another powerful erosive agent — glacial ice. The
Pleistocene period, which saw the climax of the New Zealand
mountain-building movements, was the time of the world Ice
Age. In the northern hemisphere there were four main glacial
episodes, when ice sheets a mile or more thick advanced over
northern Europe and North America. These glacial episodes
were separated by “inter-glacials”, tens or hundreds of
thousands of years in duration, when the glaciers melted away
and the climate was warm again. Enormous loads of boulders
and finer debris, carried as moraine within and upon the ice,
were deposited as thick carpets of “boulder clay” or “till”;
for example, in northern and central Europe and in Canada and
the northern United States. Long ridges of moraine mark
temporary halts in the recession of the latest ice sheets. A
study of the deposits of these glaciers suggests at least
four main Pleistocene glacial episodes, which are termed in
Europe the Günz, Mindel, Riss, and Würm (youngest), and in
North America the Nebraskan, Kansan, Illinoian, and
Wisconsin. Radiocarbon dating of charcoal fragments has shown
that this latest ice sheet (Würm, Wisconsin) began its
advance about 50,000 years ago, reached its maximum extent
about 25,000 years ago, and had retreated from Europe and
most of North America by 10,000 years ago. Although the
Pleistocene period is often stated to have begun about 1
million years ago, the age of the earlier glaciations is
uncertain.

New Zealand was of low enough latitude for its high
mountains to be heavily glaciated during the Ice Age.
Although the landscape of the South Island shows strikingly
the imprint of the latest Pleistocene glaciation, and some
relics of the one before, almost all traces of the earliest
Pleistocene glaciations have been destroyed by erosion.
Strong indirect evidence of the first onset of the chill
climate of the Ice Age is recorded, however, by fossils in
the younger marine sedimentary rocks of New Zealand; the
Pliocene-Pleistocene boundary is recognised by the
disappearance of warmth-loving shells characteristic of the
Pliocene sediments and their replacement by shells and
foraminifera that now live only in sub-Antarctic waters. An
alteration of warm and cool faunas was repeated also in
younger Pleistocene strata. Wanganui and the southern
Wairarapa are two areas where indications of changes of sea
temperature have been studied most closely. An examination of
fossil pollen preserved in Pleistocene peats and in old lake
sediments has also shown how climate fluctuated during the
Ice Age. It has been estimated that at the maximum phase of
the last glaciation permanent snow level was at least 3,500
ft lower than at present and huge areas of once-forested land
were then clothed only by tussock: the changes in the
vegetation are reflected by the changing sequence of pollen
grains. At the height of the Pleistocene glaciations,
glaciers extended from Fiordland to west Nelson, and probably
existed in the Tararua Range in the North Island.

The oldest known New Zealand glacial deposit is at Ross,
in Westland: it comprises a small accumulation of boulders
and laminated silts dating, perhaps, from the earliest
glaciation of the Pleistocene. The glaciation that produced
it has been named the Ross Glaciation: it probably coincides
in time with the first onset of cooling recorded by marine
fossils, but very little is known about the extent and
position of the glaciers of that time.

A time interval representing probably hundreds of
thousands of years separates these old Ross deposits from New
Zealand's late Pleistocene glacial deposits, formed during
two glacial stages, the Waimaungan and the Otiran. The Otiran
glaciation is probably equivalent in age approximately to the
last glaciation of Europe and America (the Würm and the
Wisconsin); the Waimaungan glaciation may be equivalent to
the Riss and Illinoian.

During the great Waimaungan glaciation more than 100,000
years ago, glaciers were thicker and more extensive in New
Zealand than at any time since. Great rivers of ice flowing
from the Southern Alps merged on lower ground to form local
ice sheets that deposited thick beds of moraine; in
south-west Nelson, for example, ice tongues joined to form a
huge glacier that advanced along the depression between
Inangahua and Greymouth, where it probably entered the sea.
On the eastern side of the Southern Alps erosion has removed
most of the Waimaungan glacial deposits, but a thick
accumulation of outwash gravels, till, and varve silts
survives at Avoca in the Waimakariri Valley. It is unlikely
that any of the landforms carved by the ice in the mountains
during this early glaciation remain.

At the time of the latest Pleistocene glaciations (Otiran)
the Southern Alps had probably attained more or less their
present elevation and their present general pattern of major
ridges and valleys: the late Pleistocene glaciers followed
these valleys, deepening and steepening them. All the
ice-sculptured landforms that add so much to the scenic
beauty of the South Island are the product of the Otiran
glaciations, particularly the latest which “ended” only about
10,000 years ago. Manapouri, Te Anau, Wakatipu, Pukaki,
Rotoroa, and almost all the other lakes of the South Island
occupy hollows scooped out by the Otiran glaciers; Milford
Sound and other fiords are deep glacial valleys drowned by
the sea; great troughs, such as the Cleddau, Hollyford, and
Arthur, owe their formation to glaciers; and Cook, Sefton,
Aspiring, and other splendid peaks of the high Alps have been
sharpened and steepened by ice. Each successive advance of
the Otiran glaciers modified the glacial features carved in
earlier advances and partly or wholly buried the earlier
moraines. Evidence of four major ice advances during the
Otiran glaciation has resulted from the study of the sequence
of moraines both east and west of the Southern Alps. The
closest studies of the Otiran glaciations have been made in
the Kumara area of Westland, in the Waimakariri Valley, and
Lake Pukaki areas of Canterbury, and at lakes Wanaka and
Hawea.

The enormous volumes of broken rock carried by the
glaciers and worn by streams from deforested mountains were
deposited on the lowlands to form broad sheets of boulders,
gravels, and sand: the Canterbury Plains were built in this
way.

How to cite this page: 'Glaciation', from An Encyclopaedia of New Zealand, edited by A. H. McLintock, originally published in 1966.Te Ara - the Encyclopedia of New ZealandURL: http://www.TeAra.govt.nz/en/1966/geology-new-zealands-geological-history/page-11 (accessed 19 Dec 2018)